The present invention relates generally to high-speed data communications, and specifically to transmission of Very High Rate Digital Subscriber Line (VDSL) signals.
Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire. DSL modems allow users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. A range of DSL standards has been defined, known generically as xe2x80x9cxDSL,xe2x80x9d wherein the various standards have different data rates and other associated features but share common principles of operation. VDSL (Very High Rate Digital Subscriber Line) is the next-generation technology in the DSL family, offering data rates up to 52 Mbit/s over short runs.
DSL modems transmit data that has been line coded (i.e., modulated) n accordance with either a single-carrier or a multi-carrier modulation scheme. Single-carrier schemes for VDSL include Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude Modulation (CAP). These schemes are described, for example, by Gitlin et al., in Data Communications Principles (Plenum Press, New York, 1992), pp. 334-347, which is incorporated herein by reference. In QAM, input data values are mapped for transmission to a sequence of symbols, each having a certain amplitude and phase. Each symbol can be represented by a complex number, which is a point in a two-dimensional xe2x80x9cconstellationxe2x80x9d of symbols. Data for VDSL transmission may be coded before modulation, using any of a variety of suitable coding schemes known in the art, or may alternatively be uncoded.
DSL transmission channels are often subject to severe inter-symbol interference, due to amplitude distortion in the frequency domain. The accepted solution to this problem is to use a decision feedback equalizer (DFE) in the receiver, in order to cancel interference from past signals. One of the problems caused by such a DFE is error propagation, since once an error has been introduced into one of the samples, the DFE will xe2x80x9crememberxe2x80x9d the error over many subsequent samples.
If the channel impulse response is known, a suitable Tomlinson-Harashima precoder can be used in the transmitter, and can eliminate the need for the DFE in the receiver. Precoders of this sort are described by Wei, in an article entitled, xe2x80x9cGeneralized Square and Hexagonal Constellations for Intersymbol-Interference Channels with Generalized Tomlinson-Harashima Precoders,xe2x80x9d published in IEEE Transactions on Communications, 42:9 (September, 1994), pp. 2713-2721, which is incorporated herein by reference. The precoder in this context is intended to compensate for interference in a channel having an equivalent discrete-time response expressed as   1  +            ∑              i        =        1            k        ⁢          xe2x80x83        ⁢                  h        i            ⁢                        Z                      -            i                          .            
The Tomlinson-Harashima precoder comprises a two-dimensional modulo device with a negative feedback loop. The modulo device takes each complex input symbol that it receives, r, into an output symbol s given by:
xe2x80x83si=rixe2x88x92kixc2x72Lxe2x80x83xe2x80x83(1)
wherein i=1,2, giving the real and imaginary parts of s and r; 2L is the modulo value; and ki is an integer such that xe2x88x92L less than s less than L. In the feedback loop, the symbols output by the modulo device are filtered by a digital filter having a discrete time response based on the equivalent discrete-time response of the channel, without the zero-order time-domain component. In other words, the filter response in the feedback loop is given by       ∑          i      =      1        k    ⁢      xe2x80x83    ⁢            h      i        ⁢                  Z                  -          i                    .      
The filtered feedback symbols are subtracted from the modulated symbols (whether coded or uncoded) that are input to the precoder for transmission.
In the receiver, the channel-distorted symbols are input to a modulo device, which is identical to that in the precoder. Assuming that the feedback filter response is well-matched to the actual response of the channel, the symbols output by the modulo device in the receiver will be identical, to within the white Gaussian noise added by the channel, to the modulated symbols that were input to the precoder for transmission. The output symbols can then be processed by a decision device or Viterbi decoder, as appropriate, to recover the input data.
U.S. Pat. No. 5,249,200, to Chen et al., whose disclosure is incorporated herein by reference, describes a device and method for combining precoding with symbol-rate spectral shaping. A data transmitter, which transmits signals to a receiver over a transmission channel, includes a Tomlinson precoding unit and a spectral shaping unit. The equivalent channel response is determined and conveyed to the preceding and shaping units, which adjust the spectral properties of the transmitted signals in accordance with the determined channel response. The precoding and shaping units may also be used independently of one another.
A further difficulty in transmitting data over twisted pair at DSL rates is that a substantial amount of radio-frequency (RF) radiation is inevitably emitted. It has been found that this emission can cause serious interference with amateur radio transmissions, particularly in the HF range. For this reason, emerging technical specifications for VDSL place strict upper limits on the radiation levels that VDSL systems are allowed to generate in HF bands that are set aside for amateur radio, such as 1.81-2.0 MHz, 3.5-4.0 MHz and other, higher-frequency bands. To meet these requirements, system designers typically add notch filters in the output circuits of their modems to attenuate signals in the forbidden frequency ranges. Such notch filters complicate the design not only of VDSL transmitters, but also of receivers. The VDSL receiver must compensate not only for distortion by the communication channel, but also for the distortion introduced in the transmitter output itself by the notch filters.
The conventional solution to this problem is to use an adaptive Decision Feedback Equalizer (DFE) in the receiver with a relatively large number of taps (together with a Forward Filter Equalizerxe2x80x94FFE). Decision feedback equalization is described, for example, in the above-mentioned book by Gitlin et al., incorporated herein by reference, pp. 500-513. The long DFE, with many taps, is undesirable for a number of reasons, including:
Error propagationxe2x80x94the longer the DFE, the longer will be the error bursts due to error propagation.
The equalizer might not converge to its optimal values, resulting in a performance loss, typically of xcx9c1 dB. Advanced adaptation methods may decrease this performance loss, but at the cost of significant additional complexity.
Slower convergence of the adaptive equalizer.
It is an object of some aspects of the present invention to provide an improved high-speed data modem.
It is a further object of some aspects of the present invention to provide methods and apparatus that enable enhanced control of the spectral profile of high-speed data transmissions.
It is still a further object of some aspects of the present invention to provide improved methods and circuitry for notch filtering of digital data transmissions.
In preferred embodiments of the present invention, a high-speed data transmitter comprises a digital transmit (Tx) filter, which filters the symbols in the transmit oath of the modem in accordance with a specified spectral profile. The profile typically includes one or more notches, such as are required for eliminating radio-frequency interference (RFI) due to the transmitter in specified, forbidden frequency bands. The symbols to be filtered by the digital Tx filter are first precoded by a Tomlinson-Harashima precoder. The precoder comprises a modulo device, as described hereinabove, and a feedback filter having a response that is substantially equal to the response of the Tx notch filter, less a zero-order time-domain component of the Tx filter response. As a result of the preceding and digital filtering, the symbols output by the transmitter have an expanded output constellation (relative to the input constellation), with a frequency spectrum in accordance with the specified profile. Frequency components in the forbidden bands are suppressed in the output.
Signals output by the transmitter are received over a communication channel by a receiver, which comprises a decision block and a modulo reduction device. The decision block preferably comprises a Forward Filter Equalizer (FFE) and a Decision Feedback Equalizer (DFE), together with a decision device. The FFE and DFE compensate for distortion in the received signals, which is generally due only to channel distortion, since distortion in the transmitter itself is substantially eliminated by the use of the Tomlinson-Harashima precoder. The decision device processes the equalized signals to generate symbols having a restored constellation that is substantially the same as the expanded output constellation of the transmitter. The modulo reduction device simply takes a modulo of each of the restored symbols in order to recover the original input symbols.
Thus, in preferred embodiments of the present invention, the Tomlinson-Harashima precoder is used in a novel fashion, as part of a digital filtering network within the transmitter itself, rather than to compensate for channel characteristics outside the transmitter. As a result of this precoding, there is no need for the DFE in the receiver to compensate for the effects of the notch filters or other shaping that must be imposed on the transmitted spectrum. The modulo devices in the transmitter and the receiver work in concord to eliminate the notch filter effects, substantially without the need for any processing parameters to be passed between them.
Transmitters and receivers in accordance with preferred embodiments of the present invention thus nave the following advantages:
No need to add taps to the DFE due to the Tx filtering. The result is lower sensitivity to error propagation and better and easier convergence of the equalizer.
Lower complexity. The precoder (like the notch filter) can be implemented with a very efficient IIR scheme, whereas DFE implementation requires FIR scheme with much greater complexity.
Simplified pulse shaping. Typically, the output of the Tx filter is passed to a pulse shape filter (typically a square root raised cosine filter). Because the Tx filter output includes the extended constellation points (which is not the case when a conventional Tomlinson-Harashima precoder is used), this output can be represented using only a small number of bits per symbol. Therefore, the pulse shape filter can be implemented in a very efficient way, using multipliers with a small number of bits. This advantage is significant, since the pulse shape filter is typically a long FIR filter.
Adaptivity. There is no need to adapt the precoder coefficients. The adaptive DFE (and FFE) take care of any changes in the channel, while the precoder handles only the constant spectral modification applied by the Tx filtere.
Interoperability. The transmitter is set to perform the required preceding and filtering without reference to the receiver. The implications on the receiver side are minor at most. No communication is required between the transmitter and the receiver during startup.
The present invention is applicable particularly to next-generation, ultra-high speed transmission systems, such as VDSL and standards under development by the HPNA (Home Phone Networking Association). It may, however, be adapted for use in substantially any transmitter/receiver pair that communicate by single-carrier modulation, whether using real or complex signal modulation schemes. While preferred embodiments are described herein mainly with reference to notch filtering, a wide range of other digital filter types may be used in the context of the present invention, including both infinite impulse response (IIR) and finite impulse response (FIR) types. The Tomlinson-Harashima precoder and filter of the present invention may also be used in conjunction with an adaptive digital filter for the purpose of xe2x80x9cwater filling,xe2x80x9d i.e., to shape the output power spectral density of a transmitter to optimally match the spectral characteristics of the communication channel.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a high-speed transmitter, which generates an output signal stream having a specified spectral profile, the transmitter including:
a Tomlinson-Harashima precoder, which is adapted to receive a sequence of input symbols having a given input constellation and to generate, responsive to the specified spectral profile, a corresponding sequence of precoded symbols; and
a transmit digital filter, which is designed to apply a transmit frequency response, in accordance with the specified spectral profile, to the precoded symbols. This filter together with the precoder generates a corresponding sequence of output symbols, having a given output constellation, to be transmitted in the output stream.
Preferably, the output constellation is expanded relative to the Input constellation, such that the input symbols in the sequence can be recovered by taking a modulo of the corresponding output symbols. Most preferably, the input and output symbols have respective real and imaginary parts, and the real and imaginary parts of the input symbols in the sequence can be recovered by taking a two-dimensional modulo of the real and imaginary parts of the corresponding output symbols.
In a preferred embodiment, the digital filter includes a notch filter, which is configured to prevent radio-frequency interference due to the output stream of the transmitter in a predetermined frequency band, as specified by the spectral profile.
Preferably, the transmitter is configured to transmit the output stream over a channel having given spectral characteristics, and the spectral profile is specified substantially independently of the spectral characteristics of the channel. Optionally, the precoder and digital filter are further configured to optimize an output power spectral density of the transmitter responsive to the spectral characteristics of the channel.
Preferably, the precoder includes:
a feedback digital filter, which is adapted to apply a feedback filter response to the sequence of precoded symbols so as to generate a corresponding sequence of feedback symbols, wherein the feedback filter response is substantially equal to the transmit filter response less a zero-order time-domain component of the forward filter response;
a subtractor, adapted to subtract the feedback symbols from the corresponding input symbols, so as to generate a corresponding sequence of subtracted symbols; and
a modulo mapping device, adapted to map the subtracted symbols to the corresponding precoded symbols, such that each of the subtracted symbols in the sequence can be recovered by taking a modulo of the corresponding precoded symbol.
Preferably, the symbols include Quadrature Amplitude Modulation (QAM) symbols, having respective real and imaginary parts, and the modulo mapping device is adapted to map both the real and imaginary parts of the subtracted symbols.
In a preferred embodiment, the output signal stream includes Very High Rate Digital Subscriber Line (VDSL) signals.
In another preferred embodiment, a receiver is adapted to receive the output stream generated by the transmitter and transmitted over a channel, the receiver including:
a decision block, adapted to process the output stream so as to substantially recover the expanded sequence of output symbols; and
a modulo reduction device, adapted to take a module of each of the recovered output symbols so as to generate the corresponding input symbol.
There is also provided, in accordance with a preferred embodiment of the present invention, a receiver, adapted to receive an input stream of digital data containing a sequence of input symbols having a given constellation, wherein the input symbols are derived from a corresponding sequence of original symbols by a transmitter that includes a Tomlinson-Harashima precoder and are transmitted over a channel, the receiver including:
a decision block, which is configured to process the input stream so as to substantially recover the sequence of input symbols from the input stream; and
a modulo reduction device, adapted to take a modulo of each of the recovered input symbols so as to generate the corresponding original symbol.
Preferably, the decision block includes:
a forward filter equalizer (FFE), which is configured to apply forward equalization to the input stream so as to generate a sequence of forward-equalized symbols;
a decision feedback equalizer, which is configured to apply decision feedback filtering to the recovered sequence of input symbols, so as to generate a corresponding sequence of decision feedback symbols;
an adder, adapted to add the decision feedback symbols to the forward-equalized symbols to generate a corresponding sequence of corrected symbols; and
an extended slicer, adapted to assign each of the corrected symbols to a corresponding value in the given constellation, thereby to substantially recover the sequence of input symbols.
Preferably, the input symbols include Quadrature Amplitude Modulation (QAM) symbols, having respective real and imaginary parts, and wherein the modulo reduction device is adapted to take a modulo of both the real and imaginary parts of the recovered input symbols. Further preferably, the input stream of digital data includes Very High Rate Digital Subscriber Line (VDSL) signals.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for generating an output stream of digital data having a specified spectral profile, the method including:
providing a sequence of input symbols having a given input constellation;
preceding the input symbols using Tomlinson-Harashima preceding responsive to the specified spectral profile, so as to generate a corresponding sequence of precoded symbols; and
filtering the precoded symbols in accordance with the specified spectral profile, so as to generate a corresponding sequence of output symbols to be transmitted in the output stream, the output symbols having a given output constellation.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: